Ophthalmic aberrometers, known also as wavefront devices (of which there are multiple types based on several different physical principles, some initially developed primarily for ground-based astronomy) and surgical microscopes are both commonly employed in corneal and cataract surgery, typically as standalone instruments. They often are not even used in the same room as one another.
Corneal or limbal relaxing incisions (LRIs) have been used for years in corneal and cataract surgery, typically without the benefit of an intraocular measurement. Such incisions can, rather than correcting the desired amount of astigmatism, produce a new and unexpected vector resultant. Surgeons utilizing recent advancements in cataract surgical lenses, such as intraocular lenses (IOL's) for correction of astigmatism (toric IOL's) and for correction of both distance and near vision (presbyopia), such as accommodating IOLs and multifocal lenses (of complex surface structures), have found it helpful to make intraoperative wavefront measurements. Use of such lenses, often known as premium lens surgery, is at a price premium and demands increased intraoperative accuracy to avoid a later, secondary surgical procedure to correct for a lack of accuracy in the first procedure. Developments in femtosecond corneal and lens-based refractive surgery for example, may also benefit from the attachment of an ophthalmic aberrometer onto an ophthalmic microscope.
Aberrometry/wavefront technology is a highly sophisticated tool for characterization of the optical characteristics of the eye and it has the possibility of providing the enhanced accuracy demanded in more sophisticated corneal and cataract surgical procedures aimed at a specific optical outcome for an eye. The concept of correcting ophthalmic aberrations dates back to at least 1987.
A standalone ophthalmic aberrometer has its own positioning mechanism to align the instrument axis with the subject eye's visual axis, and thus it is typically too heavy and too big to attach onto a surgical microscope. Also, a standalone ophthalmic aberrometer typically has a short working distance in the range of 30-50 mm, while a surgical microscope typically has a working distance of 200 mm. A short working distance makes it easier for an ophthalmic aberrometer to have a larger measurement range of defocused power. On the other hand, a big working distance is necessary for a surgeon to perform eye surgery. This allows room for the surgeon to move instruments freely and to avoid touching an object other than the eye to avoid instrument contamination. Thus, to avoid any reduction of the clearance long used by the surgeon between the eye and the microscope, a clearance in which the surgeon's long experience was garnered, and to avoid placing a constraint on the surgeon's hand(s) which could increase tension or spasm and result in an inadvertent movement or less than desirable mobility, there is a need to redesign the ophthalmic aberrometer in order to be attachable onto a surgical microscope.
Surgical microscopes are designed with high quality optical components intent on optimizing the view for a surgeon performing delicate microsurgical procedures such as corneal, cataract and other intraocular surgery. It is thus preferable that the ophthalmic aberrometer shall not interfere in any way with either the working space or the image quality/optics of the microscope.
Though the aberrometer may be fixed to the microscope and possess utility for many anterior segment surgeons, some may not use such a device and others, such as vitreoretinal surgeons, may never need such a device attached to the microscope. In these cases, interference with both clearance and optical imagery could be a disadvantage. Economic issues could preclude duplicating surgical microscopes and thus make less than ideal the operating conditions for some surgeons or prevent the anterior segment surgeon from having access to an intraoperative aberrometer.
Aberrometric/wavefront measurement parameters, such as Zernike or Fourier coefficients, can arise from different elements in the optical system, the major ones being the cornea, the natural lens or an IOL of various types. Although it is typical that the surgeon has as the aim the correction or specific alteration of the wavefront of the total eye, information on the major aberration components such as defocus or astigmatism arising from the eye as a whole or a single component such as the cornea could have significant intraoperative utility. For example, a surgical procedure such as cataract surgery typically alters the intraocular pressure from the preoperative and postoperative condition. Any deformation of the cornea by altered intraocular pressure from the fluids and viscous materials will alter the ophthalmic wavefront measurement. This information is then used to make subsequent decisions regarding lens exchange or the placement of relaxing incisions to reduce astigmatism. Should the wavefront not be taken under the expected conditions of corneal power because of an unpredicted change in the corneal curvature, the LRIs might be executed erringly. If the corneal curvatures and power are known with the additional presence of a keratometer, these values can be taken into consideration along with the wavefront measurement in the decision making process to make appropriate adjustments.